Did bioaggregates on the glacier surface trigger life seeding and pedogenesis in terrestrial environments after the Neoproterozoic Snowball Earth?

Abstract

The Precambrian time (before 540 Ma) experienced extensive and severe glaciations spanning millions of years of geological history, known as Snowball Earth. Cryogenian glaciations (720–635 Ma) were prerequisites to the Ediacaran blooming of life (635–543 Ma), and although broad attention is devoted to marine ecosystems, the debate on how terrestrial ecosystems (for instance soil formation) were built up after global glaciations is still missing. I suggest that cryoconite, a fine, dark mineral sediment mixed with organisms and covering ice surfaces around the world, could be a key factor in shaping Precambrian terrestrialization. Cryoconite on modern glaciers commonly evolves into biological aggregates facilitated by cyanobacteria. These tiny granules host aerobic and anaerobic organisms, they are biogeochemically active, store organic matter including humic substances, and are common at the glacier snout. The Precambrian scenario presented here comprises the export and deposition of biochemically active cryoconite granules to bare rocks after glacier retreat, far inland of the supercontinent Pannotia. This process started biological Neoproterozoic terrestrialization including seeding of organisms in barren forefields, weathering of parent rocks, pedogenesis, and evolution of main freshwater and terrestrial phylogenetic lineages. Observations of the ecotone between the modern glacier snout and forefield allow to reconstruct the processes of building the first terrestrial ecosystems after Neoproterozoic glaciations.